1. Field of the Invention
This invention relates to a method of fabricating Thin Film Transistor Liquid Crystal Display (TFT-LCD), and more particularly, relates to a method of fabricating a polysilicon film of TFT array in a TFT-LCD thereof.
2. Description of the Related Art
An ordinary active TFT LCD array is generally categorized into polysilicon TFT and amorphous silicon TFT based materials used for making the TFT LCD, where a polysilicon (poly-Si) TFT being capable of integrating driving circuit thus provides a higher opening rate and lower fabrication cost than a corresponding amorphous silicon (a-Si) TFT. Another reason that polysilicon TFT technology is greatly promoted is that poly-Si TFT significantly reduces device feature size so that high image resolution can be achieved. In order to mass-produce polysilicon TFT-LCD, three primary conditions are low temperature (about 450 to 550° C.) process, low-temperature filming technology for high quality gate-insulator layer, and broad ion-implantation.
In view of the cost of a glass substrate, low temperature thin film process is adopted where Solid Phase Crystallization (SPC) is introduced thereby, yet the active temperature not only tends to be relatively higher than expected, which is around 600° C., but also causes degraded crystallization. Thus Excimer Laser Crystallization (ELC) or Excimer Laser Annealing (ELA) process that is applied to the foregoing low-temperature TFT process is developed, wherein an a-Si thin film is fused by laser scanning and is crystallized to poly-Si thin film.
Providing process temperature lower than 450° C. in ELC and providing higher electron mobility and lower current leakage than SPC in forming an amorphous silicon thin film, a less expensive glass substrate is introduced so as to reduce fabrication cost whereas better TFT device characteristic is obtained thereby.
Referring to FIG. 1A, a substrate 100 is provided. A first insulating layer 102 is formed on the substrate 100. Next, a photolithography etching is performed to form a first opening 104 in the first insulating layer 102. In the sub-micron technology, the photolithography technology is not applicable to the present micro TFT field, because the threshold feature of the first opening 104 using photolithography technique is about 1 micrometer, which is relatively large compared to the threshold crystal feature size for TFT thin film.
Attempts to resolve the issue is illustrated with reference to FIG. 1B. A second insulating layer 106 is further formed over the first insulating layer 102 and the first opening 104. The deposition of the second insulating layer 106 further shrinks the first opening 104 to a second opening 108 to satisfy the feature size requirement for polysilicon TFT crystallization.
Referring to FIG. 1C, an a-Si layer 110 is formed over the second insulating layer 106. Next, fuse and liquefy the a-Si layer 110 by an Excimer Laser 112.
Finally, referring to FIG. 1D, the fused liquefied silicon undergoes crystallization from the second opening 108 to transform the a-Si layer 110 into a poly-Si layer 114, which is suitable for forming source/drain and channel of a TFT therein.
However, problems in the foregoing process do exist, as described below. The forming of the first opening 104 in the foregoing process requires a mask process and an additional deposition step of forming the second insulating layer 106 adjusting to the size of the first opening 104, and therefore not only complication but also lowers throughput results.
Moreover, the scheme of depositing the second insulating layer 106 for adjusting to the size of the second opening 108 requires precise control of the process conditions, thus narrowing the processing tolerance window.